Due to various reasons, such as laser diode deterioration, the irradiation power of a laser beam emitted from a write/read laser optics unit of an optical disc drive during the write and read processes may be closely monitored and adjusted, when necessary, to maintain the write laser irradiation power and read laser irradiation power at intended levels. Generally, the irradiation power can be monitored by a front monitor diode, or forward sensor.
FIG. 11 shows a captured computer screen display 1100 that presents a time response output voltage of an exemplary first forward sense (FS) device, e.g., a first front monitor diode (FMD), shown at 1102, in response to an output of a first laser diode, shown at 1104, driven by an optical disc drive write strategy signal. In the circuit output presented in FIG. 11, the rise time response of the first front monitor diode is sufficiently fast to achieve a peak output that corresponds with the peak power output of the first laser diode, and the fall time response of the first front monitor diode is sufficiently fast to achieve a low output that corresponds with the low power output of the first laser diode. The first front monitor diode response 1102 presented in FIG. 11 is similar to the response that may be produced by a front monitor diode within an optical disc drive in which the speed with which data is written to an optical disc media, i.e., the write strategy signal generated by the optical disc drive in response to an encoded data stream, is sufficiently slow for the front monitor diode to provide the optical disc drive with an accurate measure to the laser power output. For example, as shown in FIG. 11 at 1106 and at 1108, for each rise and fall in the laser first diode power output, there is a corresponding rise and fall in the output of the first front monitor diode in which the first front monitor diode reaches a minimum output and a maximum output in response to the minimum and maximum output of the first laser diode, respectively. In such an exemplary optical disc drive, the output of the front monitor diode may be sufficient to allow the optical disc drive to monitor, and adjust accordingly, the irradiation beam produced by a laser diode in response the power levels represented in a generated write strategy signal.
FIG. 12 shows a captured computer screen display 1200 that presents a time response output voltage of an exemplary second front monitor diode, shown at 1202, in response to an output of a second laser diode, shown at 1204, driven by an optical disc drive write strategy signal operating at an increased write speed as compared to the optical disc drive write strategy signal driving the first laser diode, shown in FIG. 11 at 1104, described above. In the circuit output presented in FIG. 12, the time response of the second front monitor diode does not have a sufficiently fast rise time to achieve a peak output that corresponds with high write power level output of the second laser diode. Such a delayed front monitor diode response may be produced by a front monitor diode in response to higher speed, higher density optical media write formats such as, for example, high-definition digital versatile disc (HD DVD), in which the write strategy frequency is increased. The rise times of front monitor diodes may be even further reduced for front monitor diodes used to monitor higher speed, higher density optical media write formats in which the write/read laser emits a narrow beam of light with a reduced wavelength, such as the blue-light laser used in optical disc drives that support optical disc formats such as blu-ray disc (BD). As further shown in FIG. 12, in optical disc drives that use higher density optical media write formats, the front monitor diode may not have a sufficiently fast fall time to accurately measure the low write power level of the laser diode.
The second front monitor diode response presented in FIG. 12 is similar to the response that may be found in an optical disc drive in which the speed with which data is written to an optical disc media, i.e., the write strategy signal generated by the optical disc drive in response to an encoded data stream, is too fast for the front monitor diode to provide the optical disc drive with an accurate measure to the laser power output. For example, as shown in FIG. 12 at 1206 and at 1208, for each rise and fall in the power output of the second laser diode, there is not a corresponding rise and fall in the output of the second front monitor diode in which the second front monitor diode reaches a minimum output and a maximum output in response to the minimum and maximum output of the second laser diode, respectively. Due to the slow rise time of the second front monitoring diode, the second front monitoring diode is not able to achieve a peak output before the output signal is forced to decay in response to a drop in the power output of the second laser diode. Therefore, the peak output of the second front monitor diode cannot be used as an accurate measure of the irradiation beam power emitted by the second laser diode. In such an exemplary optical disc drive, the output of the front monitor diode is not sufficient to allow the optical disc drive to accurately monitor, and thereby adjust, the output of the laser diode when the laser diode is driven by such a high speed write strategy signal.
Hence, a need exists for improved methods for monitoring and controlling the irradiation power emitted by an optical disc drive laser during high speed/high density optical disc media write and optical disc media read operations.